class4.dry etching

8/6/2019 Class4.Dry Etching

1/30

Dry Etching, General Principles

Dr. Marc Madou, Winter 2011

Class 4


2/30

Content

Dry etching: definition

Pressure units and modes of gasflow

Plasmas or discharges How to create a vacuum

Plasmas: DC and AC

Paschen curve

Dry etching mechanisms Dry etching types and equipment Etching profiles:

Sputtering Chemical

Ion-enhanced

Ion-enhanced inhibitor

Etching profiles in physical etching

Faceting

Ditching

Redeposition

Comparing wet with DryEtching


3/30

Dry etching: definition

Dry etching techniques are those that useplasmas (hot ionized gases) to drive chemicalreactions or employ energetic ion beams toremove material. Dry-etching processes yieldfiner patterns than wet etching (surface

tension !). These techniques also offersgreater safety as large quantities of corrosiveacids or bases are not required.

Within a dry etching reaction chamber thewafers lie directly in the plasma glow (alsocalled a discharge), where reactive ions areaccelerated towards the wafer (often biased).

The ions are a species likely to attack thesubstrate material chemically with or withoutselectivity.


4/30


5/30

Definitions of Vacuum Regimes:1) Rough Vacuum: ~0.1- 0 torr (atmospheric pressure is 0 torr)2) Medium Vacuum:~ 0.1 to 10-4 torr3) High Vacuum: ~ 10 - to 10-4 torr

4) Ultrahigh Vacuum: 10-

torr 2 modes of gas flow:

Viscous Flow regime:gas density (pressure) is high enough, manymoleculemolecule collisions occur and dominate the flow process (onemolecule pushes another). Collisions with walls play a secondary rolein limiting the gas flow.

Molecular flow regime: gas density (pressure) is very low, few molecule-molecule collisions occur and molecule- chamber wall collisions dominate the

flow process (molecules are held back by walls). See further below formathematical expressions for these two regimes.

Pressure units


6/30

How to create a vacuum

Visit on your own timehttp: et.nmsu.edu ETCLASSES vlsi files ARTICLE.HTM on vacuum pumping (take the quiz at the end).

The first thing that all basic systems have is a rough-pumping system. It is used to reduce the pressure from

atmospheric pressure in the chamber to a lower pressurelevel that other low-pressure systems can use. Then therehas to be a fine-pumping system that must be able toattain sufficient pumping speed to handle the outgassingfrom the work produced in the chamber of the vessel.There must also be vacuum gauges that determine thepressure at certain points of the system.

Pumps: Diffusion pumps operate from 10-4 Torr to 5x10-11 Torr.

Diffusion pumps operate by boiling a fluid, oftenhydrocarbon oil, and angling the dense vapor stream in adownward conical direction back into the pump boiler. Gasmolecules from the system that enter the oil curtain arepushed toward the boiler by momentum transfer from the

large fluid molecules.

Diffusion pump


7/30

Mechanical pump. Pump operation is based on bulk flow of gas; hence the pump worksin the viscous flow regime. Used for obtaining "rough" vacuum (10 -3 Torr), which is thelower limit of the viscous flow regime

Simplest plasma chamber is 2 parallel plate electrode set (anode and cathode) in alow pressure Argon filled chamber (e.g. 0.001 to 1 Torr). The two electrodes arepositioned parallel to each other, with the top electrode and chamber wallselectrically grounded while the lower electrode and substrate holder are connectedthrough a dc-blocking capacitor and matching network to a 13.5 MHz F generator(AC plasma case)

Plasmas : DC and AC

Principle of mechanical operation:(1) begin expansion cycle

(2) seal off expanded volume(3) compress gas out exhaust


8/30

Apply 1.5 kV over 15 cm--field is 100-V cm. Breakdown ofArgon when electrons transfer a kinetic energy of 15. eV to theArgon gas.

These energetic collisions generate a second electron and apositive ion for each successful strike.

If the two electrons reenergize creating an avalanche of ions andelectrons we get a glow or plasma.

At the start of a sustained gas breakdown a current starts flowingand the voltage drops to about 150 V.

To sustain a plasma, a mechanism must exist to generateadditional free electrons after the plasma generating ones havebeen captured at the anode.

The additional electrons are formed by ions of sufficient energystriking the cathode (emitting secondary Auger electrons).

This continuous generation provides a steady supply of electronsand a stable plasma.

Plates too close: no ionizing collisions (not enough energy), toofar too many inelastic collisons in which ions loose energy.

Plasmas: DC and AC


9/30

Plasmas : DC and AC

Plasma dark spaces: dark because the higher energyelectrons cause ionization rather than light-generatingexcitation.

Plasma is always positive, this follows from kinetics: fora random velocity distribution the flux of ions and

electrons upon a surface is given by:

where n is a density and v> an average velocity. Ions

are 4000 to 100,000 times more heavy than electrons sothe average velocity of electrons is much larger. Electronflux to surrounding surfaces is larger resulting in apositive charge on the plasma.

Assymetry of voltage distribution: electrons move fasteraway from the cathode than positive ions are accelerated

towards it larger space charge (also the dark space islarger at the cathode).

j i,e !n i,e vi, e

4


10/30

Plasmas : DC and AC


11/30

The largest voltage drop is in front ofthe cathode where charged particles willexperience their largest acceleration.The cathode gets etched the anode doesnot !! Substrates to be etched are laid

down on the cathode. Efficiency or strength of a particular

plasma is evaluated by the average electron energy (temperature)

ion energy (temperature) electron density (e.g. 10 and 1012 cm-3)

ion density (e.g. 10 to 1012

cm-3

) neutral species density (e.g. 1015 to 10 1

cm -3)

ion current (e.g. 1 to 10 mA cm 2).

ve ! kT e (e.g. 110eV)

v i!

kT i (e.g. 0.04 eV)

Plasmas:DCandAC

The ratio between ionized speciesand neutral gas species is 10- to10-4.


12/30

Plasmas : DC and AC

An important quantity to describe aplasma is the ratio of electrical fieldover pressure (Equation I). Withincreasing fields the velocity of freeelectrons or ions increases (~E) but an

increase in pressure decreases theelectron or ion mean free path(~1 P).The mean free path (P) is givenby Equation (II) where nv is the numberof molecules per unit volume,

The number of molecules per unitvolume, nv, can be determined from

Avogadro's number and the ideal gaslaw, leading to Equation (III)

The bombarding flux of ions on thecathode is given by Equation (VI):

kT i , e ~E

P (I)

(II)

(Equation III)

i ! qn iQ i E

(Equation IV)


13/30

Plasmas : DC and AC

d ! lnm

i

Tim

ma ! d

! T

Ema

!

AC plasmas for etching insulating surfaces.

Capacitor makes voltage distribution assymetric in this case.

A DC self bias results.

Etching energy:

Plasma energy:

Self bias:


14/30

AC Plasmas


15/30

Paschen curve

Pasc e C rve i irBreak o

oltage

Press re x ista ceP X ( mm Hg-mm)

t 1 atmosp ere = 760 mm Hg

0

0

200

400

600

800

1000

1200

1400

10 2 0 30 4 02

2.6 m 13. 16 m

Ne Physics aChemistry


16/30

Dry chemical etching mechanisms

Reactive species generation (1)

Diffuse to the solid (2)

Adsorption at the surface (3)

Reaction at the surface (4)

Reactive cluster desorption (5)

Diffusion away from the surface( )


17/30

Continuous dry-etching spectrum

low pressure 100 millitorr: physicalsputtering unselective

directional radiation damage

100 millitorr range:RIE

physical and chemical

directional

more selective than sputtering

higher pressures:plasma etching

chemical (10-1000 times faster) --see extreme example, gas phaseetching with XeF2 (not really aplasma)

isotropic

more selective

least damage

Dry chemical etching mechanisms


18/30

XeF2 Gas Phase Etching (highpressure, chemical only)

no plasma ( ust pump)

10 m min

no damage

isotropic

very selective (Si over Al,photoresist, oxide and nitride)

CM S compatible

Dry chemical etching mechanisms: purely chemical


19/30

Dry chemical etching mechanisms: Physical-chemical etching: Energy-driven anisotropy


20/30

Dry chemical etching mechanisms: Physical-chemical etching: Inhibitor-driven anisotropy


21/30

Dry etching types and equipment

Dry Etching

Glow dischargemethods-DiodeSet-up

Ion beam methods-Triode Set-ups

Plasmaetching

Reactiveion etchingsputtering

Sputteretching

Ionmilling

Reactiveion beametching

hysica etching

Ion beamassistedchem.etch

eacti egas

p asma

eacti egas

p asma

nertgas

nert gasi n

nert gasi n

eacti e gasi n eam

0.2 - 2Torr 0.01-0.2

Torr

10-4-10-3Torr

energy

m ard.

Highenergy

Highenergy

N reacti eneutra s

eacti eneutra s

mereacti e


22/30


RIE chamber with load lock

Substrate hol er for io etchi g

Substrate hol er for epositio

o e

Catho e

acuum chamber all

Matchi g et ork

13.56 MHz

F Ge

erator,1-2 kW

( F electro e ith target)

Ground shield


23/30

CAIBE RIBE IBE MIE MERIE RIE Barrel

Etchi ng

PE

Pressure

(Torr)~10-4 ~10-4 ~10-4 10-3-10-2 10-3-10-2 10-3-

10-1

10-1-100 10-1-101

Etch

Mechanism

chem/

phy

chem/

phy

phy phy chem/

phy

chem/

phy

chem chem

Selectivity good good poor poor good good excellent goodProfile anis or

iso

anis anis anis anis iso or

anis

iso iso or

anis


Reactivegas Inertgas

+++++++ +++++++Plasmasource

RIBE CAIBEReactivegas

Vacuum pump

Substrate Substrate


24/30

Acronym/Technique E lanation

AI E Chemically assistedion beam etching

ERIE agnetically enhancedreactive ion etching

IE agnetically enhanced ionetching

PE lasma etchingRI E eactive ion beam

etchingRIE eactive ion etching



25/30


26/30

Dry etching types and equipment : RIBE vs.CAIBE

CAIBE is RIE in a triodesystem (e.g. 10, 000 min)

RIBE ion is reactive and etches

(e.g. 100 min)

Re cti e s I ert s

+ + + + + + + + + + + + + +l s s rce

RIB CAIBRe cti e s

V c

str te str te


27/30

Etching profiles in dry etching

Sputtering: directional, physical.

Chemical: non-directional(diffusion).

Ion-enhanced energetic:

directional. Ion-enhanced inhibitor: directional.


28/30

Etching profiles in physical etching

Faceting: angle of preferentialetching

Ditching (trenching): sometimescaused by faceting

Redeposition: rotational stagemight reduce this effect.


29/30


30/30

Homework

1 How is a DC plasma created and how does an RF plasma differ? Why is aplasma always positive with respect to the reactor vessel walls? In whichetching setup would you prefer to etch an insulator? Is space positivelycharged?

2 Detail the different dry etching profiles available and how you obtain them.

3. Explain the DC breakdown voltage versus electrode distance curve(Paschens law) and how it is relevant to dry etching. How isminiaturization of an electrode set equivalent to creating a local vacuum?

4. Discuss the etch profiles in physical etching. Also draw profiles exhibitingfaceting, ditching, and redeposition.

5. Design a process to fabricate a polyimide post 100 m high and 10 m in

diameter on a Si cantilever. The Si cantilever must be able to move up anddown over a couple of microns.

class4.dry etching

Documents